Regeneration of magnesium bisulphite pulping liquor and absorption of sulphur dioxide during regeneration



C. E. ROGERS ETAL REGENERATION OF MAGNESIUM BISULPHITE PULPING LIQUOR AND ABSORPTION OF SULPHUR DIOXIDE DURING REGENERATION 4 Sheets-Sheet l Filed Feb. l2, 1963 INVENTORS Sept 20, 1956 c. E. ROGERS ETAL 3,273,961

REGENERATION OF MAGNESIUM BISULPHITE PULPING LIQUOR AND ABSORPTION OF SULPHUR DIOXIDE DURING REGENERATION Filed Feb. 12, 196s 4 sheets-sheet 2 START-UP STACK l l zs HoT GAS f HOT WATER MAKE-UP COLD WATER PUMP MAGNESTUM OxmE SLURRY Acro PLANT INVENTORS Charles E. Rogers Roberr E Mariy Edyvard L.Ra|s1on WW Eijk/ATTORNEY Sept 20, 1966 c. E. ROGERS ETAL 3,273,951

REGENERATION OF MAGNESIUM BISULPHITE PULPING LIQUOR AND ABSORPTION OF SULPHUR DIOXIDE DURING' REGENERATION Filed Feb. l2, 1963 4 SheetS-Sheet 5 GAs FROM Induced Draft FAN ACID To ACID PLANT 95` PRoPoRTmNrNG PUMP PRoPoRTloNxNG PUMP r j MAGNEslUM OxlDE SLURRY FROM STORAGE -WATER INVENToRs Charles E. Rogers RobenL E. MaHy Edward L. Ralsron ATTORNEY sept. zo, 1966 C. E. ROGERS ETAL REGENERATION OF MAGNESIUM BISULPHITE PULPING LIQUOR AND ABSORPTION OF SULPHUR DIOXIDE DURING REGENERATION 4 Sheets-Sheet 4 Filed Feb. l2, 1965 FIGS PER CENT SO2 ABSORPTION m Y. O

SATURATED SOLUBKUTY MMHSOB)a LIQUIDI- 407 TOTAL 502 1io LIQUID TEMPaRATURE F 3,273,961 REGENERATEN F MAGNESIUM BISULPHITE PULPliNG LIQUR AND ABSORPTIN 0F SUL- PHUR BlXiDlE DUNN@ REGENERATIGN Charles E. Rogers and Robert E. Matty, Alliance, Ohio, and Edward L. Ralston, Flossmoor, Ill., assignors to The Babcock & Wilcox Company, New York, NX., a corporation of New liersey Filed Fels. 12, 1963, Ser. No. 261,931 4 Claims. (Cl. 23-131) This application is a continuation-in-part of application Serial No. 748,024, filed luly 1l, 1958, and now abandoned.

The present invention relates to improvements in the manufacture of pulp from cellulosic fibrous materials by the acid sulphite or bisulphite processes, and more particularly to the recovery -of heat from a magnesium base cooking liquor and the reclamation of chemicals used in the pulping process.

Heretofore, a cyclic recovery system whereby chemicals are reused in the pulping process has been developed for the acid bisulphite process by the use of relatively pure magnesium base cooking liquor. When a magnesium base cooking liquor is used the residual liquor from the pulping process is concentrated by evaporation and burned with the production of magnesium oxide as a dry particle material. The magnesium oxide is separated from the entraining gases and is mixed with Water to form a magnesium hydroxide slurry and thereafter passed through absorption zones for recombination with the sulphur dioxide present in the ue gases. The resultant product provides a magnesium bisulphite cooking liquor which is very effective in the pulping of cellulosic materials.

The present invention is directed to an improvement in the system of recombining the magnesium oxide obtained by incineration of the residual liquor, with water and sulphur dioxide in the flue gas for a highly efficient chemical recovery process, wherein substantially all of the available magnesium oxide and sulphur dioxide ingredients are utilized to form the cooking liquor. The process advantageously utilizes extremely simple equipment which is inexpensive to install and operate. The apparatus forming the effective part of the absorption system includes a series arrangement of venturi-like gas acceleration zones each followed by a pressure regain section and separating chambers wherein the gases and entrained liquids containing absorbed SO2 are separated for efficient recovery of the valuable chemical ingredients contained in the liquid.

The various features of novelty which characterize our invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which we have illustrated and described preferred embodiments of the invention.

Of the drawings:

FlG. l is a diagrammatic flow sheet of a cyclic magnesium bisulphite pulp liquor recovery system embodying the present invention;

FIG. 2 is a diagrammatic fiow diagram, at an enlarged scale, showing the improved portion of the apparatus illus. trated in FIG. 1;

FIG. 3 is a diagrammatic flow diagram of another form of the invention;

FIG. 4 is an enlarged section of a portion of the apparatus shown in FiG, 3; and,

FIG. 5 is a graphical representation of the chemical absorption of SO2 from combustion gases in the apparatus of the invention.

Patented Sept. 20, 1 966 The cyclic recovery system shown in FIG. 1 is of the general character shown in U.S. Patents 2,238,456 and 2,285,876 of George H. Tomlinson. In such a cyclic system, a cooking liquor consisting of a relatively pure acid sulphite compound of magnesium, i.e., magnesium bisulphite with or without an excess of sulphur dioxide, is supplied to a digester from a cooking acid storage tank. As shown, the digester 10 is illustrated as being equipped for indirect heating so that the strength of the cooking acid is not reduced during the cooking or digestion process, as would happen in a direct heating system of digestion. By using an indirect heating digester, a more economical concentration of the residual liquor preparatory to combustion is possible.

When the cooking operation is completed, the contents of the digester 10 are discharged into a blow tank 11 from which gases are vented and the pulp and liquor mixture is pumped to suitable pulp washers 12 and 13 for the separation of the liquor from the pulp. Thereafter, the pulp is transferred to the usual pulp treating equipment for the production of the desired product.

Wash water from a hot water tank 14 is delivered to the pulp washer 13 and the filtrate discharged into a storage tank 15 from which the liquid is delivered to the other pulp washer 12 where the filtrate is used as the washing medium. The filtrate obtained from the washer 12 is delivered to an acid waste liquor tank 16 from which a portion may be returned to the tank 11 to increase the fiuidity of the pulp and to facilitate its transportation to the washers, while the remaining filtrate, or all of the filtrate, depending upon the need for the liquid in the blow tank, is discharged to multiple effect evaporators 17 for concentration. In the embodiment of the invention the liquor is concentrated to the range of 45% to 70% solids prior to its delivery to a concentrated liquor storage tank 18 for controlled introduction into the combustion zone 20 of a heat recovery apparatus or boiler 21.

As described in the referred to U.S. patents, the concentrated residual liquor delivered to the combustion zone 20 is burned in suspension and under conditions yof temperature and flow Velocity such as to insure the production of a highly reactive magnesium oxide solid. The magnesium oxide passes through the combustion zone and the associated heat absorbing surfaces of the vapor generator 21 in a gas floated condition. The gases entraining the magnesium oxide particles will contain, as major constituents, nitrogen, carbon-dioxide and sulphur dioxide. The mixture of gases and entrained solids is discharged from the heat recovery surfaces of the boiler 21 at a temperature of the order of 500 F., for example. From the boiler, the mixture of gases and solids is passed through a dust separating zone which in the present embodiment consists of a plurality of multi-clones 22. The multiclones remove substantially all of the magnesium oxide from the gases of combustion, with the gases thereafter passed through an induced draft fan 23 which discharges the gases into the SO2 recovery apparatus hereafter described.

As disclosed in said patents, the magnesium oxide particles discharge from the collecting hoppers of the multiclones, are passed through suitable Washing and treating equipment 24 for the removal of solids inert to the process, and are delivered to magnesium oxide slaking tanks 2S wherein the magnesium oxide is substantially converted to magnesium hydroxide. The magnesium hydroxide slurry is delivered to a storage tank 26. Make-up chemicals should be added at the slaking tanks so, if in the oxide form, it can be converted to hydroxide. For example, the magnesium oxide make-up may be of the order of 20 to 40 lbs. of MgO added to the system for each ton of pulp produced in the process. The magnesium addition may be added as magnesium hydroxide or may be added directly in the oxide form. In accordance with the present invention, the flue gases leaving the induced draft fan 23 are subsequently contacted with magnesium bisulphitemonosulphite solution to which magnesium hydroxide slurry has been added, `for the absorption of the SO2 in the gases and the formation of magnesium bisulphite liquor (Mg(HSO3)2) of sufficient strength for cooking purposes in the pulp digesters. The cyclic loss of sulphur in the system is -a relatively minor amount, but some make-up sulphur must ybe added. This sulphur may be conveniently added in the form of gaseous SO2 obtained from burning pyrites or elemntal sulphur.

The absorption of the SO2 which may be of the order of 1% by volume in the flue gases by the sprayed liquid of the present invention occurs by reason of the chemical ahnity of the sprayed liquid for the SO2 gas under relatively low temperature and controlled pH conditions. The sprayed liquid has, as a constituent, magnesium bisulphite which contains both free and combined SO2. The terms free SO2 and combined SO2 are used herein as defined by the Technical Section, Canadian Pulp and Paper Association (Data Sheet C- OOC, June 1955). When the bisulphite is mixed with magnesium hydroxide, monosulphite is formed with the free SO2 converted to combined SO2. Thereafter, when the mono-sulphite contacts the gaseous SO2 the gas is absorbed by the monosulphite to form magnesium bisulphite.

The apparatus illustratedin FIG. 2 includes a SO2 absorption zone wherein liquid is atomized into cooled combustion gases containing the SO2 with the mixture thereafter passed through a venturi-like accelerating zone for intimate mixing of the droplets of liquid and the entraining gas. The SO2 absorption is eiciently accomplished with a series arrangement of the venturi-like zones Where each zone is followed by a pressure regain section and a liquid-gas separating chamber wherein the liquid containing the absorbed SO2 is collected and utilized in part to provide the liquid for the absorption of the SO2. A portion of the collected liquid from each of ythe venturi-like SO2 absorption zones is passed from zone to zone in a direction countercurrent to the gas flow direction to the zones to increase the SO2 content in the acid discharged from the rst gas contact zone and is vdelivered to the acid tanks for use in the pulp digestion With the solution thereafter introduced into the SO2- con-taining gas stream, the magnesium monosulphite absorbs the SO2 to form magnesium bisulphite generally according to the formula:

MgSOa-i-Sozi-Hao Mg (H503) 2 Heretofore the amount of magnesium monosulphite present in a liquid solution has been limited by its solubility under the gas-liquid contact conditions present in the known apparatus used for SO2 absorption, for example, packed towers. When the solubility limit has been exceeded, with the liquid under relatively quiescent conditions, such as occurs in the multiplicity of relatively minute stagnant areas in a packed tower, the excess magnesium monosulphite normally precipitates in a relatively non-reactive form incapable of e'iciently absorbing gaseous SO2. It has been found that the use of the venturi type gas and liquid contact apparatus of the present invention permits the use of higher percentages of magnesium monosulphite in the liquid than heretofore without adverse precipitation, and thus makes possible correspondingly increased chemical absorption of the SO2 by the liquid.

The increased chemical absorption of the SO2 is due at least in part to the maintenance of continuous positive movement of the liquid in the liquid flow lpath from the forma-tion of the monosulphite (by the addition of the magnesium hydroxide to the magnesium bisulphite at, for example, the circulating7 pump) to its intimate turbulent spray contact with thev SO2-containing gases.

It has been observed tha-t a monosulphite concentration greater than the normal saturation solubility limit is obtainable under these operating conditions because of the formation of a suspension of microscopic and submicroscopic magnesium monosulphite crystals which are reactive with gaseous SO2 in the venturi type contact apparatus Within well defined limits of monosulphite concentration in the liquid.

The effect of solubility on absorption is illustrated in FIG. 5 where the curve A represents the heretofore known solubility limit of magnesium monosulphite content over a temperature range of F. to 120 F. in a comparatively quiescent magnesium bisulphite liquid having 4.0% total SO2. This curve lshows a substantially uniform capacity of the liquid, at saturation, to absorb only 57 to 58% of the SO2 in Contact with the liquid through the temperature range of 100 F. to 120 F. Depending upon the temperature, the magnesium monosulphite in solution may have a maximum concentration in terms of percent SO2 of from 0.3 to 0.5. More specifically, at F., the maximum soluble monosulphite concentration, expressed in SO2 percentages, is 0.5 and the liquid is capable of absorbing approximately 58% of the gaseous SO2 contacting therewith. Any increase in the monosulphite concentration above the curve A would be expected to precipitate large relatively non-reactive monosulphite crystals. The curve B represents the increased limit of magnesium monosulphite concentration attainable when used in the venturi-like absorption apparatus of the present invention. The difference between the curves A and B illustrates the increased SO2 absorptive ability of the liquid sprayed into the venturi-like apparatus in accordance with the present invention. For example, as shown by curve B in FIG. 5, monosulphite concentration of approximately 0.9% SO2 can be used at 120 F. At this value the SO2 absorption from the gases will approximate 72%.

In the venturi-like SO2 absorbing apparatus of the present invention, the absorbing liquid is sprayed into the gases, with general parallel How of the gas and liquid, i.e., co-current flow directions, and with the velocity energy of the accelerating gas being used to further atomize the liquid droplets to provide a large amount of gas and liquid contact surface area and a highly turbulent mixing of the gas and liquid. Such an arrangement provides an advantageous mechanical condition for SO2 absorption by the liquid.

As shown in FIGS. 1 and 2, the gases discharged from the induced draft fan 23 pass through a connecting duct 28 to a cooling tower 30. The gas enters the lower portion of the `cooling tower and passes upwardly through a packing of, for example, partition tile. In its passage through the cooling tower, the gases are contacted in counter-How relationship by liquid which is introduced in spray form through nozzles 31 positioned n the upper portion of the tower, and above the packing. The spray liquid eventually forms a weak acid solution of magnesium bisulphite. When the unit is initially started up, the spray liquid will consist of filtered water which is delivered through the pipe 32 from a source of filtered Water (not shown) with the spray water cooling the gases of combustion and absorbing a major portion of the residual solids in the gases. Gradually, the acidity of the spray liquid will be increased to an equilibrium point by reason of the absorption of the SO2 from the combustion gases. Moreover, the difference in temperature between the entering liquid and the flue gases will result in cooling the gases below saturation or dew point temperature with the condensation of water from the gases so that upon leaving the upper end of the cooling tower through the outlet duct 33 the flue gases may be substantially saturated at a temperature of, for example, 104 F.

To maintain the cooling effect of the liquid sprayed into the tower 30, the liquid collected in the bottom of the tower is passed through a heat exchanger 27. The apparatus disclosed consists of a series of indirect tubular heat exchange elements arranged for counter-flow of the liquid in indirect cooling relationship with cooling water. For example, with the circulation of 2270 gallons per minute of the cooling liquid utilized in the cooling tower, the liquid will enter the heat exchanger at a temperature of approximately 149 F. and leave at a temperature of 88 F.

The cooling water entering the heat exchanger may be at a temperature of approximately 60 F. and leave at a temperature of approximately 110 F. The quantites of cooling water utilized under these conditions will be of the order of 2750 gallons per minute. This heated cooling Water from the heat exchanger is passed through the connecting pipe to the MgO washing and treating portion of the system, with the excess water available for use in other portions of the plant.

The flue gases pass from the cooling tower 30 through a connecting duct 33 to an inlet in the upper portion of a gas absorption zone 34. The gas absorption zone consists of an upright closed body having a restricted venturilike throat 35 in the upper portion thereof through which the gases are passed downwardly at an accelerated velocity. A plurality of spray nozzles 36 are positioned in the upper port-ion of the zone, upstream of the venturi-throat, with each of the nozzles supplied with SO2 absorbing liquid in predetermined quantity from a connecting pipe 37 leading from a pump 38. The nozzles atomize t-he liquid which contacts the entering gases so that the liquid spray is well distributed throughout the cross-seci tional ow path of the flue gases. The acceleration of the gases in passing through the venturi throat intimately mixes the atomized liquid and the gases for absorption of SO2 from the gases into the liquid droplets. rThe lower portion of the absorption zone is provided with a gas outlet 4i) upwardly adjacent a hopper bottom 41. rthe hopper bottom is provided with a liquid outlet 42 which is connected through a pipe 43 to the suction side of a pump 45 which in turn discharges through a connecting pipe 46 leading to an acid storage tank 47 (see FIG. 2).

The gases leaving the absorption zone 34 tangentially enter the lower portion of an upwardly elongated separator 48 of cylindrical shape. The separator 48 separates the liquid droplets from the entraining gases by centrifugal force with the separated liquid collecting in the bottom of the tower which is provided with an outlet 5t) and is connected with the bottom 42 which discharges the collected liquid into the pipe 43.

The gases leaving the upper portion of the separator 48 pass into the upper end portion of a second absorption zone 51 which is a substantial duplicate, insofar as structure is concerned, of the absorption zone 34. The absorption zone is provided with a plurality of spray nozzles 52 in the upper portion thereof adjacent the flue gas inlet duct S3. The lower end portion of the absorption zone is provided with a gas outlet duct S4 connected with a separator 55 which separates the liquid droplets from the entraining ilue gases. The separator 55 is provided with a bottom outlet 56 which is connected to the suction inlet of the circulating pump 57 to recirculate liquid from the separator through a pipe 68, and the pump is arranged to discharge liquid into a connected pipe 61 leading to the spray nozzles 52 positioned in the upper portions of the absorption zone 51. The absorption zone 51 is provided with a hopper bottom having a liquid outlet. The bottoms of each of the absorption zones and each of the separators are connected by a pipe 62 which maintains a constant level in :all bodies to maintain a head of liquid `and equilibrium conditions in ilow to the storage tank 47. The gases discharge from the separator 55 through an outlet duct 63 to the atmosphere.

Since the absorption system provides recovered chemicals for forming the cooking acid used in the digestion process, during normal operation, it is necessary to provide treated make-up water to the system to maintain system rate of acid flow. Moreover, it is necessary to provide a source of SO2 gas for the system during starting up periods.

The make-up water pipe 32 is connected directly with the spray nozzles 3l, and is provided with a branch pipe 65 leading to the pipe 67 which in turn connects with both the pipe 60 and the pipe 66. With such an arrangement the cooling tower 30 may be supplied with make-up water through pipe 32 or with cooled liquid from the heat exchanger 27 through the pipe 66. In a like manner, the spray nozzles 52 of zone 51 may be supplied with 'make-up water through pipe 65 or with cooled liquid from the heat exchanger 27 through the pipe 67. The pipes 32, 65, 66 and 67 lare provided with valves so that flow through the pipes may be controlled.

To obtain eflicient absorption of SO2 in the liquid in the zones 34 and 5l, the liquid spray is controlled as to its pH value by the addition of magnesium hydroxide slurry which is obtained from the tank 26 and pumped through a closed pipe system 68. The system 68 includes a pump 70 with the return line of the system discharging into the tank 26 (as shown in FIG. 1). Suitable valved pipes 7l yand 72 connect the system 68 with the pumps 57 and 38, respectively, so as to separately regulate the pH value of the liquid delivered to the spray nozzles 52 and 36, respectively. Best results have been obtained by maintaining a pH Value of about 4.5 in the spray liquid in zone 34 `and about 5.8 in zone 51. These values are obtained by controlling the rate of admission of magnesium hydroxide slurry introduced through pipes 71 and 72. The control can be accomplished manually or automatically by regulation of valves, or by the use of proportioning pumps. It will be understood that pH values are generally indicative of the monosulphite concentration under specific conditions, such `as temperatures and acid strengths.

In the modification of the invention `shown in FIG. 3, the flue gases leaving the multiclones 22 and the induced draft fan 23 (shown in FIG. 1) pass through the duct 28 directly to the SO2 absorption apparatus of the invention. The gases are at a higher temper-ature entering the venturi-like scrubber 88 than entering the venturiscrubber zone 35 las shown in FIG. 2, and to attain efcient absorption of the SO2 gases in the magnesium bisulphiteamonosulphite solution, the liquid is cooled in an indirect heat exchanger 81 before it lis introduced into the duct 28 through the pipe 82 and the spray nozzle 83. The gases are cooled by contact with the spray liquid and are at a temperature of, for example, 120 F. in passing through the pressure regain portion of the venturi and discharging into the expansion and gas turning chamber 84. With the reduction in gas flow velocity and the abrupt turn in the direction of gas movement, liquid droplets are separated from the gases to collect in the lower inverted frusto-conical portion S5 of the chamber 84. The gases leave the chamber 84 through a duct 86 which opens to a second venturi scrubber 87 where the Igases Iare again -contacted by a spray of SO2 absorbing liquid introduced through nozzles 88 positioned in the duct 86 upstream of the throat of the venturi 87. The absorbing liquid should be cooled in heat exchanger 108 to maintain .a gas temperature of approximately 104 F. Thereafter, the gases flow through a pressure regain section 89 of the venturi scrubber 87 tangentially into a P7 `Cyclonic separator 34 where the centrifugal action of the gases separates suspended liquids, with the gases discharging to the stack 91. The collected liquid is withdrawn from lthe inverted frusto-conical bottom 92 of the separator 90.

While the venturi scrubbers S0 and 87 are shown in a vertical arrangement, the second scrubber 87 may be arranged with its axis of flow in a horizontal plane without appreciably .affecting the SO2 absorption efficiency of the arrangement. In any arrangement wherein hot gases are delivered directly to the venturi, as -in the FIG. 3 version of the invention, it is desirable to enclose the gas duct 28 with a water jacket, or the like, to avoid an accumulation of solids in the duct. Such a water jacket is illustrated at 89 in FIG. 4 where its function is to condense moisture from the hot gases to provide a wetted duct surface Ito wash the walls of the duct and to prevent deposition of solids in the duct 28 and in the entrance to the venturi. This is accomplished by encircling the duct 28 adjacent the nozzles 83 in the entrance to the venturi throat. With the duct 28 extending into the enlargement, the spray droplets will contact the wetted wall of the enlargement where the moisture is formed by condensation of moisture from the gases by reason of the cooling effect of the water jacket.

In the absorption yof SO2 from the combustion gases, the absorption liquid should be maintained at optimum oW and pH values for most eicient chemical absorption of the SO2 gases. As disclosed in FIG. 3, the liquid collected in the frusto-conical bottom 85 of the chamber 84 is passed through pipe 93 to the inlet pump 94 and through the indirect coller `81 to the pipe 82. The pH of the liquid delivered to the spray nozzle 83 is regulated by the addition of controlled amounts of magnesium hydroxide slurry added thereto through the pipe 95, as such quantity is regulated by the oper-ation of the proportioning pump 96 which is automatically regulated by a pH controller 97 positioned in pipe 82 downstream of the heat exchanger 81.

yIn a similar arrangement, the absorption liquid delivered to the nozzle 88 is withdrawn from the bottom of the cyclone separator 90 through a pipe 98 by a pump 99, passed through a heat exchanger 100 and a pipe 101 connected to the nozzle. The pH of the liquid is regulated by the addition of magnesium hydroxide slurry to the suction side lof the pump 99 through a pipe .102, as determined by the operation of the proportioning pump 103 which is controlled by a pH controller 104. Each of the proportioning pumps 96 and 103 receive magnesium hydroxide slurry from a pipe 105 which is connected with a storage source of slurry.

The excess liquid collected in the separator 90 discharges by gravity through a pipe 106 to the liquid 'collected in the bottom of the chamber 84 with the liquid in excess of the withdrawal through pump M withdrawn by a pump 107 and delivered through pipe 108 to the acid plant for reuse in the digestion of pulp.

While in .accordance with the provisions of the statutes we have illustrated and described herein the best form and mode of operation of the invention now known to us, those skilled in the art will understand the invention is also applicable to the recovery of SO2 from the gaseous combustion products resulting from the incineration of any fuels by absorption in a solution prepared from a magnesium base material. Certain features of our invention may Sometimes be used t-o advantage without a corresponding use of other features.

We cla-im:

s1. The method of treating a magnesium base residual sulphite pulp liquor which comprises concentrating the residual liquor by evaporation, burning the combustible organic constituents of the concentrated liquor in suspension to obtain a dry ash consisting mainly of magnesium oxide and combustion gases containing SO2, mixing said magnesium oxide with water to form a slurry of magnesium hydroxide, passing said combustion gases through a venturi-like SO2 absorption zone in intimate direct co-current contact with a constantly moving atomized magnesium bisulphite-monosulphite `liquid in an acid state containing magnesium monosulphite in an amount in excess of the normal solubility of the monosulphite in the liquid and at a temperature in the range between F. and the dew point temperature of the said hot combustion gases to form magnesium bisulphite in the liquid, said excess monosulphite being in the form of a suspension of reactive microscopic crystals, separating the liquid containing magnesium bisulphite from said combustion gases, discharging a portion of the magnesium bisulphite containing liquid from said absorption zone to a storage zone, withdrawing the remainder of said magnesium bisulphite containing liquid from said absorption zone, and mixing said withdrawn liquid with a controlled amount of said magnesium hydroxide slurry to form magnesium monosulphite in excess of the normal solubility of the monosulphite in the bisulphite containing liquid in the acid state while recirculating said mixed liquid to a successive absorption zone to absorb gaseous SO2 in said liquid, said excess monosulphite being in the form of a suspension of reactive microscopic crystals.

2. The method of treating magnesium base residual sulphite pulp liquor which comprises concentrating the residual liquor, burning the combustible organic constituents of the concentrated liquor in suspension to obtain a dry unsintered ash consisting mainly of reactive magnesium oxide and combustion gases containing SO2, removing substantially yall of the ash produced from the combustion zone by flotation in the gaseous products of combustion, separating the dry unsintered ash from said gaseous products of combustion and mixing said ash with water to form a slurry of magnesium hydroxide, passing said gases in series through a plurality of venturilike SO2 absorption zones in direct co-current contact with an atomized magnesium bisulphite-monosulphite liquid in yan acid state and at a temperature in the range of 100 F. to F., said liquid including magnesium monosulphite in an amount in excess of the normal solubility of the monosulphite in the liquid at the prevailing liquid temperature, said excess monosulphite being in the form of a suspension of reactive microscopic crystals, said monosulphite combining with the SO2 to form magnesium bisulphite in the liquid, separating the liquid containing magnesium bisulphite from said combustion gases leaving each of said zones, withdrawing a portion of the magnesium bisulphite liquid from each of said absorption zones for discharge to a magnesium bisulphite liquor storage zone, withdrawing the remainder of said magnesium bisulphite liquid from each of said absorption zones, mixing each of said last named liquids with said magnesium hydroxide slurry, controlling the amount of magnesium hydroxide slurry mixed with said liquids for the formation of magnesium monosulphite therein in excess of the normal solubility of the monosulphite in the particular bisulphite containing liquid, said excess monosulphite being in the form of a suspension of reactive microscopic crystals, and recirculating each of said mixed liquids to 1a corresponding absorption zone.

3. The method of treating magnesium base residual sulphite pulp liquor which comprises concentrating the residual liquor, burning the combustible organic constituents of the concentrated liquor in suspension to obtain la dry unsintered ash consisting mainly of reactive magnesium oxide and combustion gases containing SO2, removing substantially all of the ash produced from the combustion zone by tiotation in the gaseous products of combustion, separating the dry unsintered `ash from said gaseous products of combustion and mixing said ash with water to form a slurry `of magnesium hydroxide cooling said gases of combustion substantially to dew point temperature by direct contact with a cooling liquid which is heated thereby, passing said heated liquid through a cooling zone and recirculating said cooled liquid through said gas cooling Zone, passing said cooled gases through a venturi-like SO2 absorption zone in direct `co-current contact With an atomized magnesium bisulphite-monosulphite liquid in an acid state and approximately at a telnperature in the range of 100 F.-120 F., said liquid including magnesium monosulphite in an amount in excess of the normal solubility of the monosulphite in the liquid, said excess monosulphite lbeing in the form of a suspension of reactive microscopic crystals, said monosulphite combining with SO2 to form magnesium bisulphite in the liquid, separating the liquid containing magnesium bisulphite `from said combustion gases, withdrawing a portion of the magnesium bisulphite liquid from said absorption zone for discharge to a magnesium bisulphite cooking liquor storage zone, withdrawing the remainder of said magnesium bisulphite liquid from said absorption zone, treating said liquid with said magnesium hydroxide slurry to form magnesium monosulphite in excess of the normal solubility of the monosulphite in the particular bisulphite `containing liquid, said excess monosulphite being in the form of a suspension of reactive microscopic crystals, and recirculating said liquid to a successive absorption zone vfor absorption of SO2 by the conversion of magnesium monosulphite to magnesium bisul-phite in the liquid.

4. The process of absorbing sulphur dioxide contained in gases of combustion which comprises passing said gases through a venturi-like sulphur dioxide absorbing zone, spraying :an acidic magnesium bisulphite-rnonosulphite liquid `at a temperature in the range vbetween 100 F. and 120 F. co-currently into the gases entering said absorbing zone to convert said magnesium monosulphite and gaseous sulphur dioxide to magnesium bi- `sulphite, said liquid containing magnesium monosulphite in an amount in excess of the normal solubility of the monosulphite in the liquid at the prevailing temperature, said excess monosulphite being in the form `of la suspension of reactive microscopic crystals, separating the liquid containing magnesium bisulphite from the gases in a separating Zone, passing one portion of said separated liquid containing magnesium bisulphite to a storage zone, passing the remaining portion of said separated liquid containing magnesium bisulphite to the said Venturi-like sulphur dioxide :absorbing zone, mixing magnesium hydroxide slurry with said separated liquid enroute to said absorbing zone to convert magnesium ibisulphite to magnesium inonosulphite for regenerating said liquid `for absorption of sulphur dioxide therein, controlling the qurantity of said magnesium hydroxide added to said liquid for forming magnesium monosulphite therein in excess of the normal solubility limit of magnesium monosulphite in said bisulphite containing liquid, said excess n1onosulphite being in the form of a suspension of reactive microscopic crystals, yand thereby increasing the sulphur dioxide absorbing capacity thereof.

Reiierences Cited by the Examiner UNlTED STATES PATENTS 2,238,456 4/1941 Tomlinson 23-131 2,285,876 6/1942 Tomlinson 162-36 2,354,175 7/1944 Wilcoxson 23--262 2,385,955 10/1945 Tomlinson 23-131 X 2,572,929 10/1951 Hazelquist 23--430 2,893,829 7/1959 Hutton 23-48 OSCAR R. VERTIZ, Primary Examiner.

MAURICE A. BRNDISI, Examiner.

O. F. CRUTCHFELD, B. H. LEVENSON,

Assistant Examiners. 

1. THE METHOD OF TREATING A MAGNESIUM BASE RESIDUAL SULPHITE PULP LIQUOR WHICH COMPRISES CONCENTRATING THE RESIDUAL LIQUOR BY EVAPORATION, BURNING THE COMBUSTIBLE ORGANIC CONSTITUENTS OF THE CONCENTRATED LIQUOR IN SUSPENSION TO OBTAIN A DRY ASH CONSISTING MAINLY OF MAGNESIUM OXIDE AND COMBUSTION GASES CONTAINING SO2, MIXING SAID MAGNESIUM OXIDE WITH WATER TO FORM A SLURRY OF MAGNESIUM HYDROXIDE, PASSING SAID COMBUSTION GASES THROUGH A VENTURI-LIKE SO2 ABSORPTION ZONE IN INTIMATE DIRECT CO-CURRENT CONTACT WITH A CONSTANTLY MOVING ATOMIZED MAGNESIUM BISULPHITE-MONOSULPHITE LIQUID IN AN ACID STATE CONTAINING MAGNESIUM MONOSULPHITE IN AN AMOUNT IN EXCESS OF THE NORMAL SOLUBILITY OF THE MONOSULPHITE IN THE LIQUID AND AT A TEMPERATURE IN THE RANGE BETWEEN 100*F. AND THE DEW POINT TEMPERATURE OF THE SAID HOT COMBUSTION GASES TO FORM MAGNESIUM BISULPHITE IN THE LIQUID, SAID EXCESS MONOSULPHITE BEING IN THE FORM OF A SUSPENSION OF REACTIVE MICROSCOPIC CRYSTALS, SEPARATING THE LIQUID CONTAINING MAGNESIUM BISULPHITE FROM SAID COMBUSTION GASES, DISCHARGING A PORTION OF THE MAGNESIUM BISULPHITE CONTAINING LIQUID FROM SAID ABSORPTION ZONE TO A STORAGE ZONE, WITHDRAWING THE REMAINDER OF SAID MAGNESIUM BISULPHITE CONTAINING LIQUID FROM SAID ABSORPTION ZONE, AND MIXING SAID WITHDRAWN LIQUID WITH A CONTROLLED AMOUNT OF SAID MAGNESIUM HYDROXIDE SLURRY TO FORM MAGNESIUM MONOSULPHITE IN EXCESS OF THE NORMAL SOLUBILITY OF THE MONOSULPHITE IN THE BISULPHITE CONTAINING LIQUID IN THE ACID STATE WHILE RECIRCULATING SAID MIXED LIQUID TO A SUCCESSIVE ABSORBPTION ZONE TO ABSORB GASEOUS SO2 IN SAID LIQUID, SAID EXCESS MONOSULPHITE BEING IN THE FORM OF A SUSPENSION OF REACTIVE MICROSCOPIC CRYSTALS. 